gh-93607: document root attribute of iterparse

Author: Prometheus3375

Doc/library/xml.etree.elementtree.rst

@@ -628,6 +628,9 @@ Functions
    for applications where blocking reads can't be made.  For fully non-blocking
    parsing, see :class:`XMLPullParser`.
 
+   Once *source* is fully read, the returned :term:`iterator` object is populated with a ``root``
+   attribute which references to the root element of the resulting XML tree.
+
    .. note::
 
       :func:`iterparse` only guarantees that it has seen the ">" character of a